Anharmonicity of excited state potential energy surfaces: quantum chemical analysis and resonance Raman intensities

The experimental absorption and resonance Raman spectra of the radical cation of N,N-dimethylpiperazine are interpreted on the basis of ab initio density functional calculations and wavepacket propagation techniques. In particular, properties of the excited electronic state active in the resonance transition are discussed. It is shown that the excited-state potential energy surface of the radical cation is strongly anharmonic. The observed resonance Raman spectra and their interpretation using different approaches are discussed in relation to this anharmonicity. It is concluded that resonance Raman spectroscopy, in combination with quantum chemical calculations, is a valuable tool for obtaining information on possible anharmonicity of the excited-state potential energy surface.